The present invention relates to a color difference signal correction apparatus which makes a correction for changing color temperatures of brighter whites such as white telop characters in display outputs of digitally driven display devices or the like. More particularly, this invention relates to a color difference signal correction apparatus for making a correction of white characters (hereinafter referred to as white character correction).
In recent years, the demand for digitally driven display devices such as PDP (plasma display panel) and LCD (liquid crystal display) is growing, and the demand for image quality is more and more increasing. A color difference signal correction apparatus for particularly correcting color temperatures of white characters, i.e., making the white character correction has become valued as a circuit for changing a balance between primary colors of red, blue and green and correcting the color temperatures to make images more clear.
FIG. 5 is a block diagram illustrating a structure of a prior art color temperature correction apparatus for making the above-mentioned white character correction, disclosed in Japanese Published Patent Application No.2-196588. In this figure, a color signal detection circuit 54 receives color difference signals (Rxe2x88x92Y) Sf, (Gxe2x88x92Y) Sg and (Bxe2x88x92Y) Sd obtained from a color demodulation circuit (not shown) which is provided in a previous stage of the color temperature correction apparatus, and outputs the maximum value among these color difference signals as a color detection signal Sh, thereby detecting a color. An addition circuit 55 adds an input luminance signal Sa and the color detection signal Sh which is output by the color signal detection circuit 54, and outputs a corrected luminance signal Si. A slice circuit 51 compares a slice level voltage which is output by a slice level voltage generation circuit 52 with the corrected luminance signal Si from the addition circuit 55, and when the input corrected luminance signal is below the slice level voltage outputs this signal as a correction signal Sc. The slice level voltage generation circuit 52 generates a DC voltage for deciding a level at which the output of the addition circuit 55 is sliced. A subtraction circuit 53 subtracts the correction signal Sc which is output by the slice circuit 51 from the color difference signal (Bxe2x88x92Y) Sd, and outputs a corrected color difference signal. A clamping circuit 56 returns the pedestal level of the luminance signal, which has been changed due to the addition to the original level on the basis of a horizontal synchronous pulse Sj.
FIG. 6 is a circuit diagram illustrating the structure of the color signal detection circuit 54. The color signal detection circuit 54 comprises transistors 61, 62 and 63, bases of which receives the color difference signals (Rxe2x88x92Y) Sf, (Bxe2x88x92Y) Sd and (Gxe2x88x92Y) Sg, respectively, and their base resistors 64, 65 and 66. Emitters of the transistors 61, 62 and 63 are commonly connected and constitutes a color detection signal output terminal, and the transistors 61, 62 and 63 are grounded via a common emitter resistor 67. In addition,the power voltage +B is supplied to the correctors. The detected color signal is output as the color detection signal Sh from the color detection signal output terminal.
FIGS. 7(a)-7(h) are diagrams showing waveforms of signals which are obtained in the respective elements of the prior art color temperature correction apparatus. FIG. 7(a) shows the waveform of the input luminance signal Sa. FIGS. 7(b), 7(c) and 7(d) show the waveforms of the color difference signals (Rxe2x88x92Y) Sf, (Gxe2x88x92Y) Sg and (Bxe2x88x92Y) Sd, respectively. FIG. 7(e) shows the waveform of the color detection signal Sh. FIG. 7(f) shows the waveform of the corrected luminance signal Si and the waveform of the slice level signal Sb. FIG. 7(g) shows the waveform of the correction signal Sc, and FIG. 7(h) shows the waveform of the corrected color difference signal Se.
Hereinafter, the operation of the so-constructed color temperature correction apparatus will be described. Initially, the color difference signals (Rxe2x88x92Y) Sf, (Gxe2x88x92Y) Sg and (Bxe2x88x92Y) Sd having the waveforms as shown in FIGS. 7(b)-7(d), respectively, which are output by the color modulation circuit in the previous stage (not shown) are input to the color signal detection circuit 54. The color signal detection circuit 54 detects the maximum value among the three input signals and outputs the detected color signal as the color detection signal Sh. Then, the color detection signal Sh and the input luminance signal Sa as shown in FIG. 7(a) are added by the addition circuit 55 and the corrected luminance signal Si is output. In this prior art, assume that a brighter luminance signal has a lower voltage. The corrected luminance signal Si is input to the slice circuit 51 and compared with the slice level voltage Sb from the slice level voltage generation circuit 52, as shown in FIG. 7(f). Then, a part of the luminance signal, which is lower than the voltage Sb is output as the correction signal Sc as shown in FIG. 7(g). Then, the correction signal Sc is input to the subtraction circuit 53 and subtracted from the input color difference signal (Bxe2x88x92Y) Sd, whereby the corrected color difference signal Se which has been corrected to increase the color temperature of the sliced part is obtained.
As described above, in the prior art color temperature correction apparatus, the color signal detection circuit 54 detects the maximum value among the input three color difference signals and outputs the color detection signal Sh. Then, the color detection signal Sh is added to the input luminance signal Sa. Accordingly, the input luminance signal is corrected to increase its level by the color detection signal Sh in a colored part. Consequently, the input luminance signal is not sliced in the colored part and therefore even when the luminance is higher the color temperature correction is not performed to the colored part, i.e., the part which is not white.
However, in the prior art, only the maximum value among the input color difference signals (Rxe2x88x92Y), (Bxe2x88x92Y) and (Gxe2x88x92Y) is extracted by the color signal detection circuit and added to the input luminance signal, whereby the white character correction is hardly performed in the colored parts. Therefore, in some cases, the white character correction may be applied to light colors like flesh tones. In addition, it is impossible to make a fine adjustment to the white character correction with high accuracy so that the white character correction is not applied to the light colors like flesh tones.
In addition, according to the prior art construction, in cases of a picture whose frame is entirely whitish, i.e., a picture having a high brightness level of the whole frame, the color temperature correction is performed to many parts in the frame. Therefore, the sufficient effects of the white character correction cannot be obtained and further the frame becomes generally bluish.
Further, according to the prior art construction, the base resistors and the emitter resistors are used in the color signal detection circuit. The maximum value of the color is adversely varied due to dispersion errors of these resistors. Accordingly, when the luminance signal is to be shifted toward black according to the sizes of the color, there is the possibility that the signal is sliced and then the white character correction is performed with low accuracy.
Furthermore, according to the prior art construction, the fine adjustment to the effects of the white character correction so that an image which is obtained as a result of the white character correction has an optimum image quality for an apparatus which utilizes the color temperature correction apparatus cannot be made.
It is an object of the present invention to provide a color difference signal correction apparatus which can apply white character correction with high accuracy also to an image which includes a lightly colored part.
It is another object of the present invention to provide a color difference signal correction apparatus which can apply the white character correction with high accuracy also when the brightness level of the whole frame is higher.
It is another object of the present invention to provide a color difference signal correction apparatus which can prevent the reduction in accuracy of the white character correction due to dispersion in the resistors or the like.
It is another object of the present invention to provide a color difference signal correction apparatus which can finely adjust the effects of the white character correction.
Other objects and advantages of the present invention will become apparent from the detailed description and specific embodiments described are provided only for illustration since various additions and modifications within the spirit and scope of the invention will be apparent to those of skill in the art from the detailed description.
A color difference signal correction apparatus according to a 1st aspect of the present invention comprises: a first full-wave rectification means for receiving a (Rxe2x88x92Y) color difference signal and full-wave rectifying the (Rxe2x88x92Y) color difference signal; a second full-wave rectification means for receiving a (Bxe2x88x92Y) color difference signal and full-wave rectifying the (Bxe2x88x92Y) color difference signal; a first addition means for adding an output of the first full-wave rectification means and an output of the second full-wave rectification means; a second addition means for receiving a white character slice level signal, and adding the white character slice level signal and an output of the first addition means; a slice means for receiving a luminance signal and slicing the luminance signal with an output of the second addition means; and a third addition means for adding an output of the slice means and the (Bxe2x88x92Y) color difference signal. Therefore, the white character correction can be made also to images including lightly colored parts with high accuracy.
According to a 2nd aspect of the present invention, the color difference signal correction apparatus of the 1st aspect comprises: a gain control means for changing a gain of the output of the first addition means and outputting a changed gain to the second addition means, between the first addition means and the second addition means, and the second addition means adds the white character slice level signal and an output of the gain control means. Therefore, the white character correction can be finely adjusted so as not to make the white character correction to light color such as flesh tones.
According to a 3rd aspect of the present invention, the color difference signal correction apparatus of the 1st or 2nd aspect comprises a white character correction effect adjustment means for changing a gain of the output of the slice means and outputting a changed gain to the third addition means, between the slice means and the third addition means, and the third addition means adds the (Bxe2x88x92Y) color difference signal and the output of the white character correction effect adjustment means. Therefore, the effects of the white character correction can be fined adjusted.
According to a 4th aspect of the present invention, the color difference signal correction apparatus of one of the 1st to 3rd aspect comprises: a fourth addition means for receiving an average picture level signal and adding the average picture level signal and the output of the second addition means to output an addition result to the slice means, between the second addition means and the slice means, and the slice means slices the luminance signal with an output of the fourth addition means. Therefore, the white character correction can be made with high accuracy according to the brightness level of the whole frame.
According to a 5th aspect of the present invention, the color difference signal correction apparatus of the 4th aspect comprises: an average picture level gain control means for receiving the average picture level signal and changing a gain of the average picture level signal to output a changed gain to the fourth addition means, and the fourth addition means adds an output of the average picture level gain control means and the output of the second addition means to output an addition result to the slice means. Therefore, the effects of the white character correction according to the brightness level of the whole frame can be finely adjusted.
A color difference signal correction apparatus according to a 6th aspect of the present invention comprises: a first addition means for receiving a white character slice level signal and an average picture level signal, and adding the white character slice level signal and the average picture level signal; a slice means for receiving a luminance signal and slicing the luminance signal with an output of the first addition means; and a second addition means for receiving a (Bxe2x88x92Y) color difference signal, and adding an output of the slice means and the (Bxe2x88x92Y) color difference signal. Therefore, the white character correction can be made with high accuracy according to the brightness level of the whole frame.
According to a 7th aspect of the present invention, the color difference correction apparatus of the 6th aspect comprises: an average picture level gain control means for receiving the average picture level signal, and changing a gain of the average picture level signal to output a changed gain to the first addition means, and the first addition means adds an output of the average picture level gain control means and the white character slice level signal to output an addition result to the slice means. Therefore, the effects of the white character correction according to the brightness level of the whole frame can be finely adjusted.